


an elder brother like one

by ingaron



Category: Monster Girls | Monster Boys, 姉なるもの | Ane Naru Mono | The Elder Sister-Like One (Manga)
Genre: Age Difference, Alternate Universe - Gender Changes, F/M, Pseudo-Incest, Tentacles, Vaginal Sex
Language: English
Status: In-Progress
Published: 2021-03-12
Updated: 2021-03-12
Packaged: 2021-03-19 06:54:34
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,992
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29995572
Author URL: https://archiveofourown.org/users/ingaron/pseuds/ingaron
Summary: Yuka spends her summer vacation with her absolutely normal older brother.
Kudos: 1





	an elder brother like one

**Author's Note:**

> so this is a genderswap ane naru mono fic! I thought about doing regular chiyo with a fem yuu, but the concept of demon brother was too interesting to resist! even if you're not familiar with the source manga, I think you could still read this and understand!

It'd been a while since Yuka enjoyed a summer vacation like her current one. Initially, she wasn't looking forward to staying with her uncle. He lived in a small town in the mountains, but his home was still a decent walk from anything significant. It was isolated and practically on the outskirts. She planned on spending each day doing her assignment, assuming most of the locals wouldn't look her way. Her hopes were dashed even further once her uncle was admitted to the hospital soon after she arrived.

Though, Yuka's fears of a lonely summer spent house-sitting were taken care of by the arrival of her older brother, Chiyori. The charismatic man spoiled the young woman each day. She didn't have the time to worry or feel anxious.

Today, the center room played host to an unusual sight. Yuka sat alone at the table, working on an assignment, and Chiyori wasn't even home. He had gone into town to take care of some shopping. The only reason he went alone was because Yuka didn't want to fall behind on her work. While Chiyori would have loved to take her with him, he didn't want to get in her way. There was plenty of summer left, and they'd have even more time once all of her work was done. 

She had just about finished when she heard the front door opening. The sound of Chiyori calling out “I’m home!” immediately made her smile. 

Yuka closed her workbook and sat up straight, not wanting Chiyori to scold her over bad posture. The front door was nearby, so the man soon entered the room with a few bags from his shopping. Her attention was immediately drawn towards a box from the local bakery, recognizing their special packaging. 

“Oh, what did you get?” Yuka started to stand up. She was curious about what was in the box, but she also wanted to help him put away what he had bought. 

It seemed that Chiyori read her like a book and set the box down in front of her. Based on his expression, Yuka could tell that he wanted her to enjoy the sweets and let him worry about putting the groceries up. 

“You mentioned wanting something sweet the other day, didn’t you? I decided to stop by. It was the last place I went, so everything should still be fresh!” he smiled down at her. 

Yuka sat up on her knees as she lifted the lid off the box. Her honey tinted eyes widened as she saw just how many cookies and small cakes were inside. She looked between the desserts and Chiyori as if this was some illusion or trick. 

“Wow! Is it okay that you got this many..?” she asked, a bit cautious, but already reaching out to take one. 

“Well… the woman who ran the bakery actually insisted on giving me a discount, on the condition that I promised to share with my cute sister,” Chiyori spoke with a chuckle underneath his already soft voice, “And she wanted me to come by after they closed.” 

With a cookie still between her lips, Yuka turned to look up at Chiyori. Her expression shifted to wide eyed surprise. She swallowed the dessert and was about to speak up again when Chiyori grinned and lowered a finger to her lips. 

“I told her I couldn’t accept an offer like that, and I can’t leave you home alone at night either. That’d be very irresponsible of me.” 

Yuka looked relieved, and she didn’t think to hide how obvious it was. She practically yelped in surprise when Chiyori leaned forward to whisper in her ear: 

“Oh, were you jealous?” 

Chiyori’s hot breath on her neck sent a chill up Yuka’s spine. Looking in his eyes was almost too intense, so she shut her own. She was completely aware that her face was bright red too. The silence between the two of them remained for a moment, and Yuka eventually gathered enough courage to open her eyes and… Chiyori had left the room to put the groceries away. She could see his back through the doorway. All the tension in her body left through a soft exhale as she rested her head on the table. 

She had tried two more of the cookies and a small strawberry shortcake before stopping for now. There was no way she was finishing all of the sweets tonight, even if Chiyori had some as well. Yuka knew that she should get up to carry the box to the kitchen, but the heat felt like a heavy blanket on her shoulders. It must have gotten warmer or maybe she managed to ignore it while she worked. Well, if that was the case, her distraction was gone. 

With her face still pressed against the cooler surface of the table, Yuka decided to rest her eyes for a moment. Why did the heat make you tired, anyways? She grumbled the question to herself. 

Yuka could hear Chiyori’s voice from the kitchen, though he sounded muffled and distant. Leaning forward, her long hair was covering her ears and obscuring the front of her face from view. Chiyori often teased Yuka about how easy it was for her to hide away in her hair. Though, Yuka probably wasn’t paying attention to her brother since she was close to falling asleep. Her mind drifted to a familiar memory, head in his lap as he ran a hand through her hair. 

Her dream was disturbed by the feeling of something cold, hard, and wet on the back of her now exposed neck. Yuka sat up with a yelp and found Chiyori kneeling next to her. He was holding two water bottles, presumably taken right out of the fridge. He grinned at her before setting them down on the table. 

“Didn’t mean to disturb you, Yuka. Ah- that surprised look on your face is too cute, so it was worth it even if you’re a little upset…” 

She couldn’t stay mad at him for long, and she wasn’t even upset to begin with. The cold sensation was appreciated with how warm the room was getting. Yuka took the bottle into her hands, savoring the feeling of it in her hands before actually opening it. Her head tilted up to follow Chiyori as he moved behind her instead of sitting down. Before she could ask her brother what he was doing, he gathered her hair up in his hands and tied it into a ponytail. 

“Keeping your hair off your neck should make sure you don’t get too hot.” Though before Yuka could say thank you, Chiyori deviously pressed a kiss to the back of her exposed neck. 

He settled into a spot next to her. The two of them chatted about their days over the box of sweets that was still on the table. They still had some time before Chiyori wanted to start preparing dinner. Yuka couldn’t help but giggle as her brother was surprised at a cream filled pastry. 

Selfishly, Yuka hoped that days like this would last forever, but she knew that was unreasonable. Chiyori wasn’t her actual brother; he wasn’t even human. The handsome young man was actually a demon disguising himself as a human. Yuka had entered a contract with this demon, and it was his job to act like her older brother. Though, the terms on Yuka’s end were almost too vague. She knew he didn’t gain anything from spoiling her each day. She had seen small glimpses of his true power. It seemed unreasonable that a demon could be satisfied with this mundane life. 

The exact details of their first meeting were somewhat vague, and Yuka wondered if his demonic powers had anything to do with that. However, she remembered Chiyori explaining that when the time came he’d take “something precious” from her. 

Now that she had started thinking about it, Yuka couldn’t get the thoughts to leave her alone. Eventually having to part ways with Chiyori was sad, but she couldn’t ignore the guilt of potentially being unfair to him. She managed to hide her concerns while they ate dinner together, and she insisted on running her own bath to have some alone time with her thoughts. 

Worried she’d be less bold if she slept on the idea, Yuka poked her head into the spare room Chiyori had made his own. She hadn’t been inside of it often, but that was because of how he shadowed her. Even though the room was now occupied, it didn’t look that different from how it was empty. 

“Um, Chiyori?” Yuka stepped into his room. She still had her hair up in the ponytail, but was now wearing a short sleeved, cream colored nightgown. 

He looked up as if he had been expecting her. Normally he’d try to rush her off to bed, saying late nights weren’t good for her health. His light wasn’t on, but faint moonlight from the open window helped her make out the calming smile on his face. 

“Yes, Yuka?” Chiyori sat up on the bed. Yuka could swear there was a faint glow to his indigo eyes, but she didn’t question it. 

“I was thinking… I wanted to, um-” she looked down at the wooden floor and pressed her hands together as she spoke. Yuka swallowed the lump in her throat and looked directly at Chiyori, “I’m ready to fulfill the contract.” 

There was silence between the two of them. Yuka couldn’t tell if only a few seconds had passed or if it had been an hour. 

“Are you sure?” He tilted his head to the side, still smiling at her. Yuka nodded and immediately, he was in front of her. 

Though, this Chiyori was different from the one she had spent the day with. His dark hair unraveled from its own low ponytail with the ends twisting into tendrils. A pair of horns now adorned his head, and his ears now ended in points. Chiyori’s clothes were replaced by white and purple straps that really only covered the essentials and were adorned with eyes and other decorations reminiscent of fangs. It would seem that he was wearing thigh high socks and long gloves, but his skin had actually changed into the dark, inky color. 

Yuka didn’t see this transformation take place. Chiyori was human one moment, and full fledged demon the next. He took her hands with a smile before pulling her into a hug. She was turned around in his arms, so her back was facing the bed. Chiyori pulled back and thumbed Yuka’s cheek as he continued to smile.

“I’m so happy, Yuka. I hope you don’t think any less of me, but I’ve been looking forward to this for a while-” 

She closed her eyes in anticipation. He was a demon, so her mind raced as to what this precious thing could be. Blood? Her soul? Or maybe-...

Her thoughts were derailed by a kiss. Chiyori’s hands were on her shoulders as he leaned down. His lips were parted, and the tongue pressing against Yuka’s clearly tried to coax her into the same. By the time he pulled away, Yuka was breathless and a trail of drool connected their mouths. 

“W-was it… a kiss you wanted?” 

Chiyori laughed, and Yuka was a little too flustered to feel silly. He leaned in for another quick kiss before speaking again. 

“No, but you might be close if that was your first kiss.” He stepped back, and she already missed the feeling of Chiyori’s body against hers. 

“Lift your skirt up, Yuka,” he gave the order like it was the most casual thing in the world. 

Yuka raised her hands, ready to do what he asked, but she still seemed a bit confused by what he just asked. With his appearance and the moonlight hitting her back, it all felt like an odd dream. 

“You heard me.” Chiyori smiled. 

Yuka nodded. She grabbed the edge of her nightgown’s skirt and lifted it up to expose her lower body. The hunger in Chiyori’s eyes was impossible to ignore. He knelt down, now eye-level with her pelvis. His hands grabbed the soft skin of her thighs, and Yuka’s own hands trembled. 

“Purple and white stripes… Are these the ones I picked out for you?” he asked, clearly referring to her underwear. 

She wanted to bury her face in her hands from embarrassment, though that would drop the fabric of her skirt on Chiyori like an informal curtain. As if he could read her mind, two of the demon’s tendrils went up to wrap around Yuka’s wrists, holding them in place. 

His hands traveled up her thighs to the waistband of her underwear, slowly inching them down. Yuka couldn’t see what he was doing exactly, and had to guess at Chiyori’s actions based on what she felt. She didn’t see him lick his lips, but she did feel a single finger trace along her slit. 

“What I’m taking from you is… special experiences, first times, things like that~ So, all you have to do is enjoy yourself, Yuka,” Chiyori leaned forward again, pressing a light kiss below her belly button. 

“Y-yes, Chiyori…” Initially, she had just nodded, but it seemed that he wouldn’t be able to see that with her skirt blocking the way. 

The finger teasing at her entrance was soon joined by a second. Chiyori dipped them into her very slowly. He clearly savored the warm tightness around his hand and couldn’t help but wonder how she’d feel squeezing down on his dick. Restraint wasn’t something Chiyori was lacking in, though, and he knew it’d be important to warm Yuka up. 

By the time his two fingers were inside Yuka up to the knuckle, she was moaning and trembling. Her hips pressed forward, unknowingly brushing Chiyori’s fingers against a sensitive part of her depths. After a few more thrusts of his fingers, Chiyori opened them up, making a scissor-like motion inside of her. The gestures were still slow, but some drool bubbled from the corners of Yuka’s mouth in reaction. 

A pleased hum escaped Chiyori as he savored her sounds. Offering her a brief break, he pulled his fingers out. He resisted the urge to tease Yuka over the whimper that twisted out of her. Bringing the wet fingers to his lips, Chiyori licked Yuka’s off. 

“You’re so sweet, and you’re being so good for me… How about a reward?” 

Yuka could hear the smile that she couldn’t see. She didn’t have to ask about what her reward would be, as that would be answered soon enough. Chiyori’s tendrils released her wrist, which was appreciated. The demon clearly didn’t mind the fabric in his face. His hands found their place on the back of her thighs, and Chiyori buried his face between Yuka’s legs. Once again, that hot breath was driving her crazy. 

His tongue replaced his fingers, dragging along her slit before pressing inside. The now freed tendrils worked their way up her gown and stopped just above Yuka’s nipples. Her breasts were small, just barely fitting in the palm of one’s hands. The tip of one of the tendrils made circular motions around one nipple, while the other wrapped around it in a firm but still gentle pinch. 

Yuka had no experience feeling all of these sensations at once, so it was practically a miracle that she was still standing. Chiyori’s firm grip on her thighs definitely helped her stay up right. She was now somewhat grinding into his face without thinking, but her demonic brother clearly loved it. 

“I’m… I’m going to-!” Was all the warning Chiyori got before Yuka’s orgasm hit her. Yuka’s hands tightened into fists, and she tossed her head back. Her face was pointed up towards the ceiling as if engaged in some lewd prayer. 

He didn’t stop either, only pulling his tongue out to suck on her clit as he waited for her to finish cumming. Chiyori pulled away as he felt the trembling of her body ease up and the intense flow of her juices stop. The demon sat up with a grin, licking his lips once again. His tentacles gave Yuka a break, removing themselves from her clothes. 

There was no chance for Yuka to speak up again before Chiyori pulled her close and kissed her again. The tongue that had just wrenched an orgasm out of her was now pressing between her lips, making her taste herself. Chiyori’s hand came to rest on her head, and he looked at her oh-so lovingly. 

“Such a good girl, so sweet,” he cooed. 

Yuka relaxed in his arms. Most of her strength felt like it was gone from her body. She wouldn’t mind falling asleep like this but… she was surprised to feel something pressing right against her exposed core. Innocent honey eyes met indigo ones still burning with desire. 

“Brother… I’m so tired, I d-don’t know if I can do any more-” Yuka started. It wasn’t as if she didn’t want to, but she didn’t think she had it in her. 

Chiyori responded with a chuckle. Before Yuka could ask him to elaborate, a thick tentacle pressed against her lips. She was too weak to keep it shut out, and Chiyori gave Yuka an encouraging nod once she parted her lips and let it in. 

“Yuka, you forgot something. Your brother is a demon. Giving you a bit more energy to work with is easy!” He definitely could have done this without making her suck on a tendril, but it was a sight he wanted to see. Chiyori loved watching her eyes close halfway as Yuka relaxed into her mouth being used. It was a little dangerous. He was still confident in his restraint, but… if given the chance, he would throatfuck her all night. 

After a few more thrusts of the tendril, a warm liquid filled Yuka’s mouth. Her eyes opened more fully in surprise, and she immediately felt as if she had a full night’s rest. She tried to swallow all of it, but it wasn’t something she was used to. As the tendril pulled out, some of the liquid spilled out over her lips. Yuka seemed embarrassed, but Chiyori just gave her a reassuring smile. 

He found it cute. He also had to resist the urge to absolutely stuff her mouth. Would she do her best to swallow, or make a mess as she apologized? Chiyori thought about her messy face, reddened cheeks, and tear filled eyes. Ah- they needed to move along. His patience really was slipping~ 

“See? I’m sure you feel amazing!” 

Yuka nodded. The taste was sweet too, but she wasn’t sure if that was odd to say. Even though she had energy, her legs felt like jelly from how sensitive she was. Still, fulfilling the contract was making Chiyori happy! She wanted to go for as long as possible. 

A soft gasp escaped Yuka as she was lifted into Chiyori’s arms, but she wasn’t in the air for long. He set her down onto the bed, laying her onto her back in a nice pile of blankets and pillows. 

“I can finish my explanation of the contract, if you’d like~” He loomed over as his lower straps seemed to just fade away into the darkness of the room. Yuka nodded as Chiyori’s dick was finally exposed to her. It was difficult to make out the details in the low light, and she didn’t exactly have a point of reference. 

It was just… definitely big. 

“I wouldn’t expect you to be familiar with this sort of thing, but there’s a reason why so many rituals with demons require virgins,” Chiyori continued. His tendrils pulled down Yuka’s underwear completely. The skirt of her gown was pushed up far past her stomach. He settled in between Yuka’s legs, and she could feel each throb of his dick against her belly. 

Yuka’s head was spinning. She knew that length would be going inside of her, but she hadn’t actually prepared herself for that moment to come. Chiyori's explanation was heard, but it’d be generous to say she was fully understanding it. There were a lot of thoughts taking attention away from her brother’s words. 

It’s possible that even Chiyori didn’t want to go into each detail. His expression was even more hungry than before. He pulled his hips back so now the tip of his cock was pressing into Yuka’s entrance. The fluid he had her swallow might have had another effect on her body, as her pussy was practically drooling onto him. 

“Let’s just say… it’s very special that you’re letting me do this-” 

His hands found a solid grip on her thighs before he pushed into Yuka. It was slow. Chiyori didn’t want to hurt her, and he was absolutely right about her insides squeezing on down on him in the best way imaginable. It was dangerous. How irresponsible would it be if he got addicted to the feeling and insisted that Yuka sat in his lap everyday? 

Yuka writhed under him, expression shifting as she adjusted to each new inch was stuffed inside her. She breathed out his name and every other term of endearment that would apply. Once Chiyori was about halfway inside, Yuka gripped the blanket beneath her and arched her back upwards. She came crying his name to the ceiling again. 

Chiyori leaned down to capture her mouth, swallowing her moans and savoring how amazing she felt around him. Once he broke the kiss, he saw some tears beading at the corners of her eyes. She brought a hand to her own cheek and couldn’t hold eye contact for that long. 

“Sorry, I… T-that was so fast..!” Yuka apologized, but Chiyori didn’t let her feel guilty. 

“It’s okay, Yuka! You’re so cute, worrying about that!” He kissed her forehead this time. 

Yuka relaxed enough to look at Chiyori directly this time, admiring the grin on his face as he claimed her body for his own. 

“Yu~ka, I have an idea! I can cast a spell on you! If you’re embarrassed, my magic will make sure we finish at the same time. Do you like that idea?” He proposed the idea with a sweet tone, thrusting into her a bit more as he spoke. 

Still a bit too overstimulated, Yuka just nodded for her answer. 

“Such a good girl! I’m going to do something a little mean, but don’t be mad, okay?” 

Excited and emboldened by Yuka accepting his magic, Chiyori tightened his grip on her hips once again. His tendrils came to wrap around her thighs, and encouraged Yuka to hold her legs open even further. With one motion, Chiyori pushed forward to bury the rest of himself inside Yuka in one go. The demon savored the feeling of her body doing its best to adjust to the feeling, and the following cry was music to his ears. If they had neighbors, someone would have come by to bang on their door in concern. 

“Sorry! I did that to cast the spell, but you’re taking me so well, Yuka!” 

Praise was a good antidote to the immense and overwhelming sensations that Yuka was feeling. She twitched below him. Her body moved the same way it had before when she came, but there was no relief. The pressure just built up in her body even more. That was the trick to his spell it seemed. 

In a few more thrusts, Chiyori brought Yuka to the edge again, but there was no pay off. She had only just learned of this pleasure, but being denied it over and over felt too cruel. The poor girl could hardly think of anything except the dick inside of her. 

Chiyori had leaned over her completely, eclipsing her body on the bed and wrapping his arms around her back. His whispered more praise into Yuka’s ear like a chanting monk, making sure she knew just how perfect she felt around him. 

His compliments only contributed to Yuka’s now constant state of near orgasm. His magic had edged her about four times now, and she wondered just how much stamina a demon had. It was also possible that Chiyori was holding out specifically to edge his darling, but that thought was too complex. Her lust fried mind could barely entertain the idea. 

“Please… p-please, Chiyori..! Let me cum! I’ve been good! Please..!” Yuka begged. She cried. She pleaded. 

Chiyori was a lot of things, but he wasn’t cruel enough to ignore begging that was so desperate. It was also a bit selfish. He wanted to see her expression as four orgasms hit her body all at once. The demon also wanted to cum inside her. 

“You’re so cute Yuka! Of course you can cum~” He kissed her again. Chiyori sped up his thrusts, knowing their actual climax would be sitting soon enough. He released Yuka from their embrace so he could hold her face in his hands. 

“When I release my spell… I’m going to cum inside you too. I’ll stuff your pussy completely and fill your womb too. Are you okay with that, Yuka?” He asked like it was an actual warning, as if she’d actually say no. 

Yuka nodded desperately, as if her life depended on it. “Yes, please, please, please Chiyori..!” 

Pressing her firmly into the bed, Chiyori released his spell. His dick throbbed, pumping thick, demonic cum into Yuka just like he promised. The intense orgasm that rocked Yuka’s body had her insides practically sucking on his length, as if she was subconsciously milking him for his seed. Did humans have an instinct for that sort of thing, or was it just her? Either way, it made her body even more enjoyable. 

The poor thing was practically sobbing as she came her brains out. Chiyori’s hands found Yuka’s, and she held down on them for dear life. He noted the squeeze was less tight than the pleasurable vice he found his lower half in- 

Each time her body seemed to wane and calm down, there was another peak waiting for her. Chiyori pressed kisses all over her face and even whispered “I love you” a few times into Yuka’s ear. He wasn’t surprised that she fell asleep before he even got the chance to pull out. The demon made a note to be a bit nicer with that spell going forward. 

After he pulled out, Chiyori knew cleaning up would be easy, but he savored the few of his cum slowly dripping out of Yuka. That combined with her sweet, sleeping face… it took all of his self control to not take her again. He’d be a terrible brother if he overindulged.

Yuka woke up in Chiyori’s bed. That wasn’t too surprising. There were times when they stayed up late watching movies, and she regularly fell asleep before the climax. He’d often take her to bed with him when that happened. 

As she sat up, Yuka tried to remember how she got there and immediately recalled everything she and Chiyori did the previous night. Well, not everything. Some of the memories were blurry hazes, obscured by an almost lustful pink mist. A blush crept onto her face. For a moment, she wondered if it was just a really vivid dream, but that would make less sense. 

She sat up straight as Chiyori entered the room with a smile. He was in his human form again. 

“Glad to see you’re awake, Yuka!” he leaned over the bed and pressed a kiss to her forehead. 

Yuka wanted to smile at him, but a look of concern started to cross her face, “Um, Chiyori… last night, d-did we…?” 

Before she could finish the question, Chiyori interjected, “We fulfilled the contract. You don’t have to embarrass yourself, Yuka.” 

He sat down next to her on the bed, putting an arm around her shoulder and pulling her close. Yuka relaxed in his arms fairly quickly. It was tempting to fall back asleep, but there was clearly something still on her mind. 

“Well, about that..! I wanted to ask you some things.. about it.” When she initially spoke up, Yuka’s voice was bold, but she settled into a flustered tone as she continued. 

Chiyori grinned at her, “Ask anything you like, darling. I’ll answer whatever questions you have.” 

“If we… fulfilled the contract, are you going to leave?” Yuka looked down at her hands in her lap. Her voice wavered as some dread set in. Everything that happened last night was intense, but she initiated it because she knew she needed to accept Chiyori leaving her one day. She was surprised to hear him chuckle, and looked up only to be greeted with a soft kiss. 

“You’re too cute. I’m a powerful demon. Just one session, won’t completely satisfy our contract.” He wrapped his other arm around Yuka and joined her under the blanket. In one swift motion, Chiyori pulled Yuka into his lap. “Besides, I hate the idea of leaving my cute sister all alone~” 

Yuka nodded. She was smiling, and it was impossible to hide it. Tilting her head to look at Chiyori directly, she spoke up to ask her second question. Even with the happiness from knowing he’d be staying for a while, she spoke in a low tone again. 

“Um… l-last night was… risky, right? Is anything going to… happen?” The question was phrased like she didn’t really understand what she was asking, but Chiyori understood immediately. He couldn’t help but laugh. 

“Oh dear, darling! That’s such a bold question!” Chiyori squeezed Yuka a bit tighter and kissed her forehead again. “You don’t have anything to worry about. Consider it a term of our contract. It’ll only happen… if you want it to.” A firm hand rested on her stomach. Yuka tensed in her arms. 

“Now then, what did you want your brother to make you for breakfast?”

**Author's Note:**

> I hope you enjoyed this! It was a loose adaption of the first doujin. I plan on writing more, not doing 1:1 adaptations of the doujins or the manga, but I'll definitely take inspiration from the later doujins while including my own ideas. If there's any scenarios you'd like to see, please leave suggestions in the comments!


End file.
